PSJJJJ is meant to be unpronounceable, symbolizing that any attempt to categorize Change Itself would, by virtue of its adherence to an UNchanging word, fail to adequately describe what it attempts to describe. PSJJJJ is not a true word in the scrict sense...it is an attempt to understand something that cannot possibly be understood, state of mind one can achieve but for a moment before what one is attempting to understand changes yet again.
I could probably say it, it just wouldn't sound very good. Like blowing a raspberry.
But there was once a mystery cult that added Daedric principles falling within the Sphere of Mephala in an attempt to provide them with something that could be consistently worshipped in some form of structured manner. ending up with "Sithis", an odd conceptual mixture of PSJJJJ and Mephala. These people became known as the Dark Brotherhood. The blessings and Wrath of Sithis is, more than likely, a manifestation of the Webspinner's daedric might in the guise of "Sithis".
Why would Mephala do this? Who knows, it is the Webspinner after all.
Yours in the Scrolls,
___The Word Merchant of Julianos
I know this part is somewhat off-topic, but the Dark Brotherhood aren't the only to claim to worship Sithis. The Crimson Scars also worshipped him, and even had a working altar to him. Sithis may indeed be a sentient entity.